Competitive adsorption of methane over carbon dioxide on modified activated carbon

Abstract

The use of natural gas as a fuel has become an attractive alternative to gasoline and diesel fuels because of its inherent clean burning characteristics. Adsorbed natural gas (ANG), in which natural gas is adsorbed by a porous adsorbent material at a relatively lower pressure with similar methane capacity than commercially compressed natural gas (CNG), has gained much attention. Carbon-based adsorbents, like activated carbon, could provide high adsorption capacity and delivery due to its high specific surface area and high volumetric storage capacity. In this work, the adsorption capacity of methane was investigated by using coconut shell activated carbon (CSAC) as an adsorbent having high surface area (952 m2/g) and total pore volume (0.52 cm3/g) in a packed bed reactor at atmospheric pressure and room temperature. Equivolume mixture of methane and carbon dioxide was used. Gas adsorption was determined by GC. The adsorbent was treated by strong and weak alkali solutions to clean pores and modified with methyltriethoxysilane (MTES), which is a hydrophobic promoter. Results showed that carbon dioxide significantly affected the adsorption of methane. The breakthrough time of carbon dioxide was longer than that of methane because carbon dioxide was more selectively adsorbed on all adsorbents than that of methane. The amount of carbon dioxide adsorption decreased with the increase in the MTES amount due to the hydrophobic extent on the surface. The Langmuir isotherm was used to fit the experimental data. The relative error of predicted data was less than 20%.